I think that part of the answer lies in the values we subscribe to both politically and as individuals, personal or individual freedom being the foremost of them. Other political mainstreams may also believe in freedom - but they have 1) a different understanding of freedom and 2) it is of lower priority in their system of values.
Another part of the answer is that liberalism is not an ideology in the same sense as conservatism or socialism is. A lot of what liberals want to achieve has little to do with attaining some kind of utopia. Instead, liberals try to get things done that are within our reach. In the process we use our common sense and maintain a sense of proportion and realism.This is perhaps less ambitious - but a more successful approach as experience shows.
A third important point concerns the liberalism commitment to the use of reason whenever we are looking for solutions.